The present exemplary embodiments relate to photobioreactors. They find particular application in conjunction with optimizing the illumination of photobioreactors by combining a solar collector light guide system with LED lighting, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiments are also amenable to other like applications.
Photobioreactors are enclosed culture vessels designed for controlled biomass production of phototrophic liquid cell suspension cultures, such as algae. Photosynthetic organisms, like algal biomasses have several rapidly growing applications such as, for example, use as an energy source, food supplements, cosmetic additives, pigment additives, and pollution control agents. Algal biomasses are also very useful in the production of biofuel, a typically non-toxic and biodegradable environmentally safe alternative to conventional fuel.
Photobioreactors offer many advantages over open systems, such as preventing or at least minimizing contamination, offering better control over cultural conditions (pH, carbon dioxide, temperature), preventing water evaporation, lowering carbon dioxide losses, and permitting higher cell concentrations. For successful cultivation, algae needs light, nutrients and mixing, while shear forces and high oxygen levels should be avoided. Algal culture systems can be illuminated by artificial light, solar light, or both. A difficult feature for photobioreactors to provide is an even distribution of light at a proper intensity throughout the bioreactor. Generally, in most systems, ideal light intensities occur only in a very small part of the bioreactor. Often, inside the culture, light cannot penetrate because of cell shading and microalgae are subjected to darkness. The photobioreactor walls may experience dramatic drops in photosynthesis yield due to photoinhibition and heat dissipation caused by too high light intensities. Photoinhibition is the light induced reduction in the photosynthetic capacity of a plant, algae, or cyanobacterium.
Many different types of photobioreactors exist, such as tubular reactors, flat panel reactors, vertical column reactors, and bubble column reactors. Typically, tubular photobioreactors are widely used for the mass cultivation of algae. Most tubular photobioreactors are usually constructed with either glass or plastic tubes and can be in the form of horizontal/serpentine, vertical, conical, or inclined, for example, to maximize sunlight capture. Tubular photobioreactors are preferred for outdoor mass cultures of algae since they have large illumination surface area. However, photoinhibition is very prevalent in tubular photobioreactors when the reactors are scaled up by increasing the diameter of the tubes, since the illumination surface to volume ratio decreases. Additionally, the productivity of the photobioreactor is limited to the intensity of the sun, which itself depends on the time of day, season, the localization and the diurnal cycle. It is possible to provide an artificial light to compensate for the periods of low intensity. However, the use of artificial light can be very costly and requires a large amount of energy usage. Therefore, it may be desirable to maximize crop yield by combining concentrated solar energy with photovoltaic cell-battery powered LED lighting to maintain optimum light level for biomass production throughout the day.
Several photobioreactor designs have been developed to try to harness solar energy. U.S. 2009/0047722 discloses including, within a bioreactor system, a solar energy system that collects and/or supplies sunlight, as well as direct light into the bioreactor. The solar collector is coupled to the lighting system, which comprises a network of fiber optic waveguides and optical switches to route, guide, and eventually emit at least a portion of the light collected by the solar collector toward at least some of the algae within the bioreactor. The reactor is illuminated from the exterior surface. U.S. Pat. No. 6,603,069 teaches a full spectrum solar energy system that uses a hybrid solar concentrator that collects, separates, and distributes the visible portion of sunlight, while simultaneously generating electricity from infrared portion of the spectrum. The disclosed system also implements large-core polymer optical fibers to deliver large quantities of visible sunlight into photobioreactors.
Additionally, photobioreactors have been created for use solely with artificial light sources. U.S. Pat. No. 5,614,378 describes a photobioreactor system for a closed ecological life support system including, for example, an optical transmission system, uniform light distribution, and continuous cycling of cells. The optical transmission system illuminates the reactor internally and includes a light source which is external to the reactor preventing heat generation problems. The light source may by any conventional light source and filters may be utilized to eliminate undesired wavelengths.
Generally artificial light sources in photobioreactors consist of high pressure sodium (HPS) and/or fluorescent lamps. However, these light sources are problematic for plant cells. HPS lamps must be placed a safe distance from the algae to avoid over-lighting and potential burning of the plant cells. Additionally, although HPS lamps provide high light output, the lighting level is not uniform along the tube. Most significantly however, is that both HPS and fluorescent lamps produce light on wavelengths which plants cannot absorb to use as an energy source.
Therefore, a need exists for a bioreactor design that can combine a solar collector-light guide system with a photovoltaic cell-LED lighting system to fully optimize the illumination level and distribution.